Various mechanical and electromechanical instruments are currently used for measuring acceleration, inclination, velocity, and motion, including piezoelectric and piezoresistive instruments, and force balanced, capacitive or convective accelerometers.
In accelerometers having a force feedback or servo-accelerometers the inertial mass is spring-suspended between two permanent magnets and can move between these permanent magnets. The displacement of the mass due to external acceleration is measured by standard electrodynamic methods. A signal of a sensor is amplified and the resulting current passes through a coil wound on the mass, thereby producing a rebalancing force that restores the inertial mass to its original position. The accelerometers of this type have high sensitivity and accuracy; however, they also have a high cost.
Another type of accelerometer which is capable of measuring an angular velocity is based on the phenomenon of injection of gas into a chamber through a nozzle under the effect of external acceleration. The chamber has two sensing elements in the form of wires arranged so that the injected gas is uniformly distributed between the sensing elements in the absence of external acceleration. In the presence of an acceleration, the gas accumulates near one of the wires, which becomes colder than the other wire. The difference in the resistance of the two sensing elements is proportional to the angular velocity. A main disadvantage of such an accelerometer is the presence of a spraying nozzle, which makes the instrument bulky and expensive.
Another type of accelerometer is a convective accelerometer. An example of a prior convective accelerometer is one that contains a heating element installed at the center of a housing and two temperature sensing elements arranged in the housing symmetrically with respect to the heating element. In the absence of external acceleration, a heated gas circulates symmetrically with respect to the heating element and the temperature sensors are essentially at the same temperature so that the difference of their readings is close to zero, thereby indicating the quiescent state. In the presence of external acceleration this symmetry is broken, and the sensing elements are at different temperatures. The respective temperature difference is then proportional to the external acceleration. The disadvantages of such an instrument include a low dynamic range, low sensitivity, and high consumption of energy due to the presence of a heating element. Such an accelerometer is also incapable of measuring purely rotational motion.
Thus, there is an urgent need for highly sensitive accelerometers having a wide frequency and dynamic range, small size, low power consumption, low weight, and low cost.